Patent GB-1073715-A (GB)
Title: Improvements in nuclear reactors
IPC: G21C1/18
Filed: 1964-06-18
Abstract: 1,073,715. Reactors. BABCOCK & WILCOX CO. June 18, 1964 [June 18, 1963], No. 25189/64. Heading G6C. A nuclear reactor includes a pressure vessel having means therein defining a coolant fluid flow path within the pressure vessel including a plurality of tubes arranged for coolant flow in parallel therethrough surrounded by moderating material spaced from the pressure vessel wall, means defining a gaseous moderating fluid flow path within the pressure vessel separated from the coolant fluid flow path, nuclear fuel elements arranged within the tubes in heat transfer relationship with the coolant fluid, means for passing coolant fluid through the coolant fluid flow portion and means for passing through the moderat ing fluid flow portion a gaseous moderating fluid having a controlled concentration of isotope. Such a reactor is illustrated in Fig. 2 in which the pressure vessel 10 housing the reactor core includes an elongated cylindrical shell portion 66 closed at its lower end by a hemispherical head portion 68 having an outlet nozzle 70 and terminating at its upper end in a closure flange 72 to which is secured the closure flange 76 of an upper closure head 74 provided with an inlet nozzle 80. A cylindrical member 86 supported from a groove 84 in the inner surface of flange 72 has a horizontal tube plate 88 across its lower end while a hemispherical member 96 supported from the nozzle 70 by an outlet connection 102 has a horizontal tube plate 98 across its upper end, a multiplicity of tubes 104 extending vertically between the two tube plates 88, 98. The outlet connection 102 includes a plurality of concentric tubular members formed of materials having differing coefficients of thermal expansion, adjacent pairs of which are connected alternately at their upper and at their lower ends, so forming an expansion compensator to compensate for differences in vertical thermal expansion between, on the one hand, the pressure vessel 10 and, on the other hand, the pressure vessel internal assembly of the cylindrical member 86, the tubes 104 and the hemispherical member 96. The reactor fuel is arranged within the tubes 104 in the form of fuel elements 114 which are suspended from the upper tube plate 88; Layers 116 of neutron moderating material, e.g. graphite, are arranged around the space occupied by the tubes 104 to provide a reflector and a layer 118 of thermal shielding material is arranged between the outer circumferentially surrounding layer 116 and the shell portion 66 of the pressure vessel 10. Further layers 120 of thermal shielding material are disposed below and above, respectively, the upper and lower tube plates 88, 98. Moderating gaseous fluid inlet nozzles 124 are provided in the wall of the nozzle 70 for the introduction of moderating fluid into the annular space between the nozzle 70 and the outlet connection 102, which then flows upwardly around the hemispherical member 96 to the upper surface of the lower tube plate 98. Channels (not shown) extend through the upper and lower horizontal reflector layers 116 and the thermal shielding layers 120 to permit the moderating fluid to flow upwardly in the space through which the tubes 104 extend. The tubes 104 may be provided with thermally insulating material to minimize the transfer of heat from the coolant fluid flowing therein to the moderating fluid outside the tubes. The moderating fluid then flows upwardly adjacent the outer surface of the cylindrical member 86 before flowing downwardly adjacent the pressure vessel 10 and also through flow passages (not shown) in the outer circumferential layer 116 and the layer 118 to outlet nozzles 126 in the lower head portion 68. Suitably sized subsidiary outlets 128 are provided in the lower extremity of the nozzle 70 for the flow of moderating fluid through the annular space in this portion of the nozzle to maintain its temperature within desired limits. A plurality of tubes 130 form a circular row around the tubes 104, extending through the tube plates 88, 98 and over the inner surface of the hemispherical member 96 to a header or headers at the upper end of the outlet connection 102, their purpose being to supply a flow of only slightly heated coolant fluid from the header or headers through tubes delivering to the spaces under coolantpressure between adjacent tubular members of the expansion compensator, thence into the coolant outflow, so controlling the temperature of the temperature compensator. The coolant fluid circulated through the reactor may be any one of many possible suitable fluids, e.g. steam or other suitable vapour or gas, pressurized light or heavy water, organic fluids, liquid metals and molten salts. The moderating fluid is a single-phase gaseous fluid having a hydrogen-isotope content, preferably capable of substantial density changes with changes in enthalpy, and may include superheated steam, vaporous hydrocarbons, hydrogen gas or mixtures thereof or mixtures thereof with inert gases. The hydrogen-isotope atoms may be all hydrogen or all deuterium or some hydrogen and some deuterium. A reactor as described above may be operated and its output controlled by the use in the moderating fluid flow portion of the space within the pressure vessel of steam at a temperature of about 700‹ F. and a pressure of about 3000 p.s.i. By varying the density of the steam within the core, the reactivity of the reactor may be controllably varied, as for example to change power level, while if light water steam is diluted with heavy water steam, by changing the ratio of the two steams, the moderation may be changed over the life of the fuel. The reactor shut-down operation may be accomplished by discharging steam through a relief valve which may be either automatic or selectively operable as conditions dictate so that upon the reception of a " scram " or emergency shut-down signal from the reactor control system in the case of a selectively operated valve, or an increase of the moderator steam pressure above a predetermined value in the case of a pressure relief valve, the valve opens, venting the moderator circuit to a lower pressure receiver. The reactor possesses negative coefficients of reactivity with moderating fluid specific volume and temperature so that reactor power may be read ily increased by introducing a quantity of moderating fluid into the moderator circuit, while at the same time flow through the coolant circuit is increased to maintain the desired coolant outlet temperature. If the required moderator pressure becomes unduly high, the hydrogen content of the moderating fluid may be appropriately increased, reducing the moderating fluid pressure required for a required amount of moderation. In the reactor described and using steam as both coolant fluid and moderating fluid, the reactor will have a negative coefficient of reactivity with coolant enthalpy. The reactor dispenses with the provision of neutron-absorbing control rods such as one commonly used for suppressing excess reactivity, for controlling reactivity during normal working and as emergency shut-down means.